Anisotropic magnets produced by milling crystalline, magnetically anisotropic materials such as ferrites or rare-earth alloys and pressing the milled material in a specific magnetic field are widely used in speakers, motors, measuring instruments and other electrical devices. Of these, because magnets with anisotropy in the radial direction in particular are endowed with excellent magnetic properties, are freely magnetizable and require no reinforcement to fix the magnet in place as in the case of segment magnets, they are used in AC servomotors, DC brushless motors and other related applications. The trend in recent years toward higher motor performance has brought with it a demand for elongated radially anisotropic magnets.
Magnets having a radial orientation are manufactured by vertical pressing in a vertical magnetic field or by backward extrusion. Vertical pressing in a vertical magnetic field is characterized by applying opposing magnetic fields through the core of a mold in the pressing direction so as to obtain a radial orientation. That is, as shown in FIG. 1, a magnet powder 8 packed into a mold cavity is radially oriented by means of a magnetic circuit in which magnetic fields generated by orienting magnetic field-generating coils 2 are applied toward each other through cores 4 and 5, pass from the cores through a die 3, and circulate back through the press frame 1. Also shown in FIG. 1 are a top punch 6 and a bottom punch 7.
Thus, in this vertical magnetic field-generating vertical-compacting press, the magnetic fields generated by the coils create a magnetic path consisting of the cores, the die and the press frame. To reduce magnetic field leakage loss, a ferromagnet, and primarily a ferrous metal, is used as the material making up the portions of the press that form the magnetic path. The strength of the magnet powder-orienting magnetic field is set by the following parameters. The core diameter (magnet powder packing inside diameter) is represented below as B, the die diameter (magnet powder packing outside diameter) as A, and the magnet powder packing height as L. Magnetic fluxes which have passed through the top and bottom cores meet from opposite directions at the core center and move on into the die. The amount of magnetic flux that passes through the core is determined by the saturation flux density of the core. The saturation magnetic flux density in an iron core is about 20 kG. Therefore, the strength of the orienting magnetic field at the magnet powder packing inside and outside diameters is obtained by dividing the magnetic flux which has passed through the top and bottom cores by, respectively, the inside surface area and outside surface area of the region in which the magnet powder is packed, as follows:2·π·(B/2)2·20/(π·B·L)=10·B/L (inner periphery);2·π·(B/2)2·20/(π·A·L)=10·B2/(A·L) (outer periphery).Because the magnetic field is smaller at the outer periphery than at the inner periphery, to obtain good orientation in all areas of the packed magnet powder, a magnetic field of at least 10 kOe is required at the outer periphery. As a result, 10·B2/(A·L)=10, and so L=B2/A. Given that the height of the powder compact is about one-half the height of the packed powder and is reduced further during sintering to about 80%, the magnet ultimately obtained has a very small height. Because core saturation determines in this way the strength of the orienting magnetic field, the size (i.e., height) of the magnet that can be oriented is dependent on the core shape. Manufacturing cylindrical magnets that are elongated in the axial direction has thus been difficult. In particular, it has been possible to manufacture small-diameter cylindrical magnets only to very short lengths.
The backward extrusion process for manufacturing radially oriented magnets is not conducive to the production of low-cost magnets because it requires the use of large equipment and has a poor yield.
Thus, regardless of which method is used, radially anisotropic magnets are difficult to manufacture. The inability to achieve the low-cost, large-volume production of such magnets has in turn made motors that use radially anisotropic magnets very expensive to manufacture.
Recently, owing to a strong desire by manufacturers for lower material and assembly costs, there has been an urgent need to improve the productivity and ease of assembly for radially anisotropic ring magnets as well. On top of this, product miniaturization and labor-saving trends have also created a desire for higher magnet performance. It is believed that elongated radially anisotropic ring magnets can satisfy such requirements by manufacturers. Here, “elongated” is used to refer to ring magnets whose length is greater than the inside diameter.
When such a magnet is achieved by stacking a plurality of short magnets, a number of problems arise. That is, the magnet and the motor core are bonded together with an adhesive or by the magnetic forces of attraction between the magnet and the ferromagnetic motor core. However, when the adhesive fails, because the force of attraction between the magnets is greater than the force of attraction between the magnets and the core, the north poles and south poles on adjacent magnets bond to each other. As a result, the motor ceases to function. Moreover, even when the adhesive has not failed, the forces that try to pull the magnetic north and south poles toward each other create shear stresses on the adhesive that encourage it to fail. By contrast, in a one-piece magnet, such forces do not arise. Even should the adhesive happen to fail, because the magnet and the ferromagnetic motor core are mutually attracted by magnetic forces, they do not separate.
Radially anisotropic ring magnets are manufactured by vertical pressing in a vertical magnetic field as shown in FIG. 1, yet this conventional process is only capable of producing short magnets. A method for producing radial magnets which are elongated bodies of integral construction is disclosed in JP-A 2-281721. However, this prior-art publication describes a multi-stage molding process in which a starting powder that has been filled into a die cavity is magnetically oriented and pressed to form a compact. The compact is transferred to a non-magnetic portion of the die, and the cavity in the magnetic portion of the die that opens up as a result is filled with more starting powder, which is then pressed. The resulting compact is likewise transferred downward. Powder feed and pressing are repeated a desired number of times in this way to obtain an overall compact having a large dimension L in the axial direction of the ring (referred to hereinafter as the “length”).
Radially anisotropic ring magnets of substantial length can indeed be manufactured by multi-stage molding. However, this process involves repeatedly feeding and pressing powder, causing joints to form in the powder compact. In addition, the long molding time required to produce a single multilayer powder compact makes such a process unsuitable for mass production. Moreover, the load applied during pressing of the compact is constant, and so sintered bodies obtained from the resulting compacts of uniform density tend to develop cracks at the joints in the powder compact. JP-A 10-55929 discloses a way to reduce crack formation at joints in the powder compact by setting the density of the compact during multi-stage molding to a value of at least 3.1 g/cm3 in the case of Nd—Fe—B-based magnets, and carrying out a final pressing operation (the compact obtained by final pressing being called-herein the “final compact”) such as to result in a compact density at least 0.2 g/cm3 higher than the density of the compacts obtained up to that point (referred to herein as “preliminary compacts”).
However, this method requires strict pressure control. Moreover, because the condition of the magnet powder varies considerably depending on the particle size and particle size distribution of the magnet powder and the type and amount of binder, the optimal pressure differs each time, making the pressing conditions difficult to set. In addition, if the preliminary compacts have a low density, they are subject to the influence of the magnetic field during the second and subsequent pressing operations, resulting in poor magnetic properties. If the final compact has a low density, cracks form at the joints. On the other hand, a final compact with too high a density will result in disruption of the orientation during final pressing. It is thus exceedingly difficult to manufacture by the foregoing process elongated radially anisotropic ring magnets in such a way as to achieve both good magnetic characteristics and a good yield.